This invention relates to electrophotographic development utilizing magnetic toner particles which are applied from magnetic brush to the electrostatic latent image without use of a carrier material.
There have been suggested systems for magnetic development not utilizing the carrier material. One such system was described in U.S. Pat. No. 2,846,333 to Wilson. Wilson et al disclosed the use of magnetic brush to apply toner particles formed of ferrites and resin material to develop electrostatic latent images. The difficulty with this process was the conductivity of the toner made electrostatic transfer difficult.
Another process utilizing magnetic toner involves the development with a magnetic toner of magnetic images carried on a surface such as magnetic tape. Such a process is illustrated in U.S. Pat. No. 3,804,511. One drawback of such a process is that the heavily, about 65 percent by weight loading magnetite of the toner particles results in a toner image which is difficult to fuse particularly at a high rate or at normal fusing temperatures.
A further development of magnetic development without carriers is illustrated by Kotz, U.S. Pat. No. 3,909,258 wherein an electrostatic development process utilizing a magnetic brush without carrier is illustrated. A toner suitable for use in the Kotz process is disclosed in U.S. Pat. No. 3,639,245 to Nelson wherein a dry toner powder having specific electric conductivity is disclosed. The toner of Nelson is formed by blending magnetite with the resin and then after blending pulverizing to a small particle size. The particles are then mixed with conductive carbon black which is imbedded in the surface of the particle and then a small particle size SiO.sub.2 agent is mixed into the toner to improve the flowability. The toner of Nelson suffers from the disadvantage that it does not transfer well from a photoconductive substrate to plain bond paper.
A method of forming toners that are field dependent such that they will develop conductively and transfer electrostatically has been invented and described in copending patent application Ser. No. 792,636, now abandoned entitled "Single Component Magnetic Toner", which is hereby incorporated by reference. This technique provides excellent development and transfer characteristics. However, the toner is formed by the spray drying techniques that result in magnetite concentrated at the surface of the toner giving higher fuse temperatures than are preferable to utilize present roll fusing techniques.
Therefore there is a continuing need for toners suitable for use in one component magnetic development systems. Further there is a need for toners suitable for high speed development and the resultant high speed fusing to plain paper. There is further need for toners which will flow well and transfer effectively. There is a continuing need to lower fusing roll temperatures to conserve energy and lower cost.